1. Field of the Invention
This invention relates to the conversion of an aqueous thiosulfate solution to a mixture of carbonates and sulfides in aqueous solution which is suitable for use in connection with the scrubbing of flue gases to remove SO.sub.2. The thiosulfate may be sodium, potassium, or ammonium thiosulfate.
2. Description of the Prior Art
The conversion of aqueous thiosulfate to a mixture of carbonates and sulfides according to the following equations is well known. EQU M.sub.2 S.sub.2 O.sub.3 + 4 CO + 2 H.sub.2 O = M.sub.2 CO.sub.3 + 2 H.sub.2 S + 3 CO.sub.2 ( 1) EQU m.sub.2 s.sub.2 o.sub.3 + 4 co + 3 h.sub.2 o = 2 mhco.sub.3 + 2 h.sub.2 s + 2 co.sub.2 ( 2) EQU m.sub.2 s.sub.2 o.sub.3 + 4 co + h.sub.2 o = 2 mhs + 4 co.sub.2 ( 3)
where M is Na, K, or NH.sub.4. Other sulfides, e.g. M.sub.2 S and M.sub.2 S.sub.x may also be formed.
The reduction of aqueous M.sub.2 S.sub.2 O.sub.3 according to the above expressed reactions is inexplicably variable and unpredictable in liquid phase at elevated temperatures under most conditions. The rate of reaction is unpredictable and generally undesirably slow, with a resulting inordinately long residence time.
The reduction of aqueous M.sub.2 S.sub.2 O.sub.3 by means of formate, MOOCH, where M may be Na, K, or NH.sub.4, may be expressed by the following reactions: EQU M.sub.2 S.sub.2 O.sub.3 + 4 MOOCH = 2 MHS + M.sub.2 CO.sub.3 + 2 MHCO.sub.3 + CO.sub.2 ( 4a) EQU CO.sub.2 + M.sub.2 CO.sub.3 + H.sub.2 O = 2 MHCO.sub.3 ( 5)
this reaction is relatively fast, and the rate is significantly increased with increase in temperature. Accordingly, in the flue gas scrubbing process described in U.S. Pat. No. 3,690,824 wherein the spent absorbent contains M.sub.2 S.sub.2 O.sub.3 and MOOCH as the major constituents, the formate is used to reduce the thiosulfate to carbonates and hydrosulfide as set forth in equations (4a) and (4b).
In certain flue gas scrubbing processes, the spent absorbent contains M.sub.2 S.sub.2 O.sub.3 as the principal constituent but there is no formate, or at most, only a minor amount of formate present. Illustrative of patents showing such spent absorbents are British Patent No. 1,389,372 and U.S. Pat. No. 3,859,416. No advantage is taken in these processes of the rapid rate of reaction between formate and thiosulfate to form carbonates and sulfides which are used as SO.sub.2 absorbent and sulfite-reducing agent respectively in such processes.
Patents which contain certain features of the present invention include the following: Br. Pat. No. 1,389,372; U.S. Pat. No. 3,687,615; U.S. Pat. No. 3,584,042; U.S. Pat. No. 3,690,818; U.S. Pat. No. 3,592,850; U.S. Pat. No. 3,690,824; U.S. Pat. No. 3,649,183; U.S. Pat. No. 3,718,691; U.S. Pat. No. 3,687,614; U.S. Pat. No. 3,859,416.
The term "carbonate" as used herein and in the claims, means either M.sub.2 CO.sub.3 or MHCO.sub.3, unless otherwise so indicated. The term "sulfide" as used herein and in the claims, means MHS, M.sub.2 S, M.sub.2 S.sub.x, or mixtures thereof, unless otherwise so indicated.